Method for disturbance-free operation of at least two base stations in a universal mobile telecommunications system

ABSTRACT

A method for disturbance-free operation of at least two base stations working according to the CDMA (Code Division Multiple Access) principle in a universal mobile telecommunications system, preferably in-house base stations, with partially overlapping radio fields relative to one another and/or with respect to a telecommunications radio cell network, wherein, the base stations transmit the communications information using the CDMA multiple access method, at least two participating stations carry out an autosynchronization procedure for the purpose of eliminating disturbing interferences on account of the overlapping radio fields and identical transmission frequency, chip synchronism being produced as a result and, consequently, good reception of the orthogonal codes, i.e. good subscriber separation of the multiple access method, is ensured.

The present invention relates to a method for disturbance-free operationof at least two, preferably in-house UMTS (Universal MobileTelecommunication System), base stations with partially overlappingradio fields in a telecommunications radio cell network, the basestations transmitting communications information using the CDMA (CodeDivision Multiple Access) multiple access method, and also to a basestation and to a mobile station for disturbance-free operation in auniversal mobile telecommunications system.

DESCRIPTION OF THE PRIOR ART

It is known, in wire-free communications systems, to utilize physicalchannels for the transmission of messages or data. By the utilization ofsuch physical channels, the communications information, for example inthe form of real time video voice data or recently real time data, iscommunicated via an air interface from a first station to a secondstation. In the case of duplex radio connections, information is alsoexchanged in the opposite direction via the same air interface.

Parameters of the physical channels are, for example, a specific timeslot in a TDMA (Time Division Multiple Access) radio communicationssystem, a specific carrier frequency utilized during the communicationof the communications information in an FDMA (Frequency DivisionMultiple Access) radio communications system, and a specific code usedto code the communications information for radio transmission in a CDMA(Code Division Multiple Access) radio communications system.Combinations of the known multiple access methods TDMA, FDMA and CDMAare also possible.

In a known mobile radio system, in particular the so-called GSM system(Global System for Mobile Telecommunication), the allocation of theradio channels via which communications information can be transmittedbetween a specific base station and a specific mobile part is performedcentrally by a coordination unit. The coordination unit assigns aspecific number of radio channels to the individual control units of theGSM base stations taking account of specific country conditions (e.g.,the interference situation calculated a priori).

Furthermore, radio communications systems exist which work in so-calleduncoordinated operation. In systems of this type, the radio channels arenot allocated centrally for the entire system, rather the radio stationsparticipating in a radio connection that is to be set up select forthemselves the relevant channels from an existing channel supplyaccording to specific criteria. One example of a radio station whichworks in the uncoordinated operation mentioned above is that whichoperates according to the DECT standard.

Thus, in known radio communications systems, in particular in a GSM, asystem according to the DECT standard or in the future UMTS, duplexradio connections are set up in order to bidirectionally transmit dataeven at relatively high data rates, voice information or, generally,communications information of other services via a radio interface.Known methods include here, in particular, the so-called TDD (TimeDivision Duplex) method, in which a first radio channel and a secondradio channel of the same duplex radio connection utilize different timeslots of the same carrier frequency.

The third-generation mobile radio system UMTS is intended to cover boththe conventional so-called outdoor range and in-house range. For theoptimal functioning of a mobile radio system of this type, the envisagedresource planning is inherently provided, and explicit frequencyplanning is not necessary. While a handover to the adjacent base stationis made under severe interference conditions (adjacent base station is asource of interference) in the outdoor range, this cannot happen within-house base stations. Therefore, in this case an effective method ofinterference elimination (synchronization→midamble→interferences) iscrucial for a high spectral efficiency of the system. In the case of amobile radio system which covers both the quasi-public and the privatesectors, the difficulty that arises for the operator is that home basestations installed in the private sector are not under the control ofthe operator and, therefore, cannot be controlled from the network.

In the case where, on account of such home base stations becomingincreasingly widespread, such home base stations are located in directproximity, which is the case for example in a multiple dwelling, theradio fields of the individual home base stations can then overlap, withthe consequence of reciprocal interference and of limited orinterference-impaired radio transmission.

In the DECT structure, that channel which allows an undisturbedconnection set-up for data transmission is selected in a quasiself-organizing manner by the subscribers of the respective localnetwork before the connection set-up. In the case where interferenceoccurs, a channel changeover is made to a channel which is once againfree from interference or freer from interference, such changeovergenerally being imperceptible to the user. Since the radio communicationis realized under TDMA in the case of the DECT system, a channelchangeover is not very problematic even in the absence ofsynchronization between the subscribers; i.e., in the event of achangeover to another base station.

Difficulties arise, however, when the CDMA multiple access method isintended to be used for a universal mobile radio system. In that case,the chip synchronism, i.e. the use of mutually orthogonal CDMA codes, isa prerequisite for preventing interference caused by the users or inorder to obtain optimal spectral network efficiency. If a number of basestations are operated asynchronously in such a system, severeinterference and, under certain circumstances, considerable losses ofcapacity shall be registered on account of the lack of orthogonality ofthe CDMA codes; particularly when, even in the in-house range,high-bit-rate data services are intended to be handled or taken up viathe system.

Ep 0 865 172 A2 discloses a method for operating at least two overlaidwire-free communications systems, the two communications systems beingat least one indoor and one outdoor communications system and, forinterference-free operation of the two systems, existing radioconnections in the in-house range being handled via time slots inaccordance with a time division duplex method and, at the same time,these time slots being utilized in order to monitor radio channelsutilized for existing radio connections in the outdoor range, with theresult that rapid reaction to changing traffic requirements or reactionto interferences is possible.

It is an object of the present invention, therefore, to specify a methodfor disturbance-free operation of at least two in-house UMTS basestations with partially overlapping in-house radio fields in atelecommunications radio cell network, the assumption being that thebase stations transmit communications information according to the CDMAprinciple; i.e., multiple access method. The present invention isintended to increase the spectral efficiency through optimal utilizationof the network conditions, without the need for reserving a broadfrequency spectrum specifically for the in-house application. At thesame time, the present invention is intended to improve the signal ordata transmission quality, so that even in a region of high localdensity of the base stations, there is sufficient channel capacityavailable for data transmission.

SUMMARY OF THE INVENTION

Accordingly, the method-pertaining basic concept of the presentinvention consists in the fact that the at least two participating basestations which have partially overlapping radio fields perform anautosynchronization procedure for the purpose of eliminating disturbinginterferences precisely on account of the overlapping radio fields andthe fact that it is no longer possible to fall back to two frequencies.Chip synchronism is achieved as the result of this autosynchronizationprocedure, and it is thus assured that orthogonality of the codes of themultiple access method of the participating, adjacent base stations isproduced.

In a preferred embodiment of the present invention, a reserved in-housefrequency range in the UMTS system is assumed, in which case, during thetransmission of a base station in such a reserved in-house frequencyrange, the additional further base station firstly checks whether thedesired, selected frequency is free or busy. In the case where theadditional base station selects a busy frequency, a fallback is effectedto a free frequency in the reserved frequency range or frequency band.

When simultaneous operation of a multiplicity of base stations ispresent, so that complete capacity utilization of the reserved frequencyrange occurs or such a consequence arises with the added base station,the respective additional station selects a minimally interferingfrequency or a frequency having the smallest interference component anddetects the base station accordingly. Afterward, chip synchronism withrespect to this detected base station or with respect to the nearestbase station is produced in accordance with the CDMA stipulations.

The autosynchronization procedure proposed according to the presentinvention differs from standardized synchronization procedures forcoordinated operation, which are controlled centrally, by virtue of adecentralized mode of operation.

In other words, an initiation and control of the synchronizationprocedure is performed by each base station within its own cell. Thissynchronization is preferably suitable for uncoordinated multicellsystems and produces the required synchronism of the participating basestations.

According to the present invention, for the autosynchronizationprocedure, a reserved synchronization channel SCH with synchronizationsignal is provided in the transmission frames transmitted by the basestations, the signal, upon reception by the added or additional basestations, enabling the production of chip synchronism between theparticipating stations. When chip synchronism is produced, the addedbase station is able to receive the synchronization signal in a reservedrandom access channel RACH.

The reserved SCH is regularly a downlink channel and the reserved RACHan uplink channel, which are alternately transmitted at a periodicinterval.

In the case where further additional base stations are to besynchronized, synchronization is effected with the synchronizationchannel having the highest received field strength or transmission powerin accordance with a time slot pattern.

According to the present invention, the base stations may have aninternal random number generator which initiates, for the respectivebase station, a function exchange or allocation exchange of the timeslots for the synchronization channel and the random access channel, asa result of which renewed synchronization is possible even in the eventof failure of the synchronization reference station.

Finally, it is possible for each base station to transmit on thesynchronization channel or on another channel information about the CDMAcodes already used in the cell. This information can be received by therest of the participating base stations, but it also can be recieved bythe mobile stations, and be utilized in connection with a channelestimation—carried out using a midamble for the purpose of minimizing oreliminating the intracell and the intercell interferences, the resultbeing improved spectral efficiency.

After the conclusion of the synchronization procedure between the basestations, in a manner known per se, a synchronization with respect tothe mobile part is performed, so that after the secondarysynchronization has been effected, the connection set-up for the purposeof transmitting and exchanging messages can be effected.

A further aspect of the method of the present invention consists in thefact that user or base station detection is possible by each basestation transmitting a specific midamble. A channel estimation ispossible using the transmitted midamble, the channel estimationestimating the transmission channels between the mobile station and upto 8 in-house base stations. This channel transfer function is then usedfor determining and eliminating the interference signals (interferenceelimination). In this case, the time-slot and chip synchronism is anessential prerequisite for a usable channel estimation result. Inprinciple, the spectral efficiency in the respective local radio cellcan thereby be improved.

The base station proposed according to the present invention fordisturbance-free operation in a universal mobile telecommunicationssystem has a receiving device for detecting further active systems inthe relevant in-house frequency range. A channel or frequency selectionassembly that is provided makes it possible to change or to select achannel or a given frequency in the reserved frequency range. Thesynchronization procedures are handled via a synchronization assemblywhich can take the form of hardware and/or software. Specifically, thesynchronization assembly enables the selection of time slots assuringfreedom from interference or of codes during the communication ofmessages toward the mobile part.

In an embodiment an interference analysis unit is implemented in therespective base stations in order, when the allocation of all availablereserved frequencies or channels has been ascertained, to select afrequency or a base station with minimal disturbance or interference, inorder then to produce the required time-slot or code synchronismtherewith for traffic handling purposes.

Via the method according to the present invention and the associatedbase station, the possibility can be created, through the principle oftime-slot synchronization, wherein a number of home base stations areallowed to transmit at the same frequency, but in different time slots,without reciprocal interference occurring. The chip-exactsynchronization of the stations that is achieved furthermore enables ahigher spectral efficiency in such a way that a number of mobile partswhich are respectively assigned to different base stations can workwithout interference at the same frequency in the same time slot, theCDMA components being optimally utilized in this case. It is thuspossible to open up, in the in-house range, applications with high-ratedata transmission quantities, without having to use the public network.In terms of present method, then, synchronization of home base stationsis achieved locally, i.e. outside the sphere of influence of a centrallycontrolled resource planning, wherein good reception of the orthogonalcodes is ensured.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Preferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a possible scenario in which two in-house or home basestations potentially interfere with one another due to the givenreception conditions within a building, with and consequence ofcorresponding interferences; and

FIG. 2 illustrates frame structures of the participating base stations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In principle, it is assumed that a separate frequency band withcorrespondingly reserved frequencies (f_(n) . . . f_(N)) is madeavailable for UMTS applications in the local area. The thenquasi-superordinate cellular UMTS mobile radio applications are handledin the frequency range f₁ . . . f_(n−1).

Taking the reserved frequency range as a departure point, in the case ofthe two base stations 1 and 2 which are situated in a building 5 and areshown by way of example, a check is now made prior to the connectionset-up, utilizing a selected frequency, to determine whether possiblythis frequency is not already busy. For this purpose, the base stations1 and 2 have corresponding receiving devices for detecting furtheractive systems and also a channel or frequency selection assembly whichcan be used to change or select a channel or a frequency.

If the base station to be activated ascertains that the selectedfrequency is already busy, a fallback is made to a frequency that isstill free in the range (f_(n) . . . f_(N)), or to a corresponding otherchannel.

In the case where the adjacent base stations are active on allfrequencies available in the range f_(n) . . . f_(N), i.e. there is nofree frequency available, the additional base station checks theinstantaneous interference characteristics and determines that frequencyat which only minimal or the least interference is received. Thecorresponding detection of these frequencies and, thus, of the basestation communicating there is followed by the initiation ofsynchronization with the aim of chip synchronism.

FIG. 2 illustrates frame structures of the participating base stationsfor the autosynchronization. The designations chosen in FIG. 2 for thebase stations BS1, BS2 et cetera at the same time serve for illustratingthe order of the autosynchronization procedures to be carried out.

The starting point is formed by the operational base station BS1,completely allocated adjacent frequencies being presupposed here.

The base station BS2 would now like to be synchronized with the basestation BS1 and receives the frame structure as illustrated. In thiscase, the base station BS2 seeks a special signal for thesynchronization channel SCH, which can be used to produce chipsynchronism with the base station BS1. The base station BS2 is nowlikewise able to receive the synchronization channel SCH of the basestation BS1 on a so-called random access channel RACH.

The RA channel may be the same channel used by a mobile station in orderto request at the base station the set-up of a telephone connection. Thechannels RACH and SCH are, in this case, special reserved channels, SCHbeing a downlink channel and RACH being an uplink channel, which aretransmitted alternately at a specific periodic interval.

In the example shown, the period is equal to 2, RACH and SCH beingtransmitted once every two frames, by way of example. In this case, aframe corresponds to 16 time slots.

If a further base station, for example the base station BS3 is added,then the latter seeks the synchronization channel or channels SCH havingthe highest transmission power and is synchronized with this time slotpattern in a chip-synchronous manner, as shown in FIG. 2. Then, with thetransmission power of the base stations BS1 to BS3 being received at thesame level, an additional base station BS4 would be synchronized withthe base stations BS1 and BS3, etc.

In the case where the transmission power of the base station BS2 exceedsthe sum of all other synchronization channels (base station BS1 and basestation BS3) and the base station BS4 is newly added, the base stationBS4 would be synchronized with the base station BS2. In the case of afunctional failure of the base station BS2, e.g. as a result of networkinterruption, the remaining base stations BS1, BS3 and BS4 would nolonger receive a reference synchronization signal; namely,synchronization channel from BS2. Therefore, appropriate precautionsmust be taken for stable operation in such a scenario. For this purpose,an instant in which the function of the time slots of thesynchronization channel and random access channel are interchanged iscommunicated to each individual base station via an internal randomnumber generator. Assuming that the base station BS1 would be the firstto interchange the position of RACH (at present slot 16) and SCH (atpresent slot 32) in the time slots, the result of this is that RACH(afterward slot 32) and SCH (afterward slot 16) are transmitted, so thatthe base stations BS3 and BS4 can again receive a referencesynchronization signal, as a result of which the system is stable.

Furthermore, each base station can transmit, on the synchronizationchannel or a further channel information about CDMA codes already usedin the respective cell. This information can be received by the otherbase stations, but also may be recieved by the mobile stations, and beutilized in connection with the channel estimation carried out using themidamble for the purpose of reducing or eliminating the intracell andintercell interferences, thereby establishing an improvement in thespectral efficiency. With regard to FIG. 2, it also shall be noted thatBCCH denotes the broadcast control channel. After autosynchronizationhas been effected, i.e. on achieving the use of orthogonal CDMA codes ofthe participating stations, in a manner known per se, thesynchronization with respect to the corresponding mobile part isperformed and the connection set-up is effected.

The handling procedure described enables chip-synchronous operation ofadjacent base stations and of corresponding mobile parts, correctorthogonality of the CDMA codes precluding reciprocal interference andenabling optimal utilization of the frequency spectrum.

In the case of UMTS devices utilizing an air interface with a TD-CDMA(Time Division—Code Division Multiple Access) standard, the time slotsynchronization can be used to achieve the situation where a number ofparticipating base stations transmit at the same frequency in differenttime slots, without reciprocal interference occurring. Furthermore, itis possible for a number of mobile parts assigned to different basestations to be active without interference at the same frequency in thesame time slot, with the result that the CDMA-inherent advantages andcomponents can be optimally utilized.

In accordance with FIG. 2, the method sequence is realized by thehardware components, taking account of the reserved frequencies f_(n) .. . f_(N) which lie outside the frequency range f₁ . . . f_(n−1) of theUMTS base station 6, as follows.

In a first step, a synchronization known per se and a call set-upbetween the first mobile part 3 and the first base station 1 areeffected. If the second base station 2 is to be activated and bothstations 1 and 2 transmit at the same frequency and no further freefrequencies are available, synchronization of the base station 2 withthe base station 1 is performed. In a next step, the second mobile part4 is synchronized with the second base station 2. After the selection ofan as yet unallocated channel in accordance with the CDMA code or of acorresponding time slot, the call set-up between the mobile part 4 andthe base station 2 can then be initiated, without interference occurringdespite identical transmitting and receiving frequencies of theparticipating base stations 1 and 2.

Attention shall be drawn to the fact that just two base stations and twomobile parts have been illustrated in the drawing merely for reasons ofsimple illustration. It goes without saying that in the context of theavailable codes and taking account of the spreading factor and also inaccordance with the frequency range f_(n) . . . f_(N), a significantlylarger number of base stations can be operated without interference indirect proximity, without departing from the principle described by thepresent invention.

Indeed, although the present invention has been described with referenceto specific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

What is claimed is:
 1. A method for disturbance-free operation of atleast two base stations working according to a Code Division MultipleAccess (CDMA) principle in a universal mobile telecommunications system,each of the at least two base stations having a radio field whichoverlaps with at least one of the radio field of the other base stationand a telecommunications radio cell network, the method comprising thesteps of: performing an autosynchronization procedure to eliminatedisturbing interferences based on overlapping radio fields and identicaltransmission frequencies, wherein a reserved synchronization channelwith a synchronization signal is provided in transmission framestransmitted by each respective base station; producing chipsynchronization between the base stations and orthogonality of the codesof the multiple access method upon reception of the respectivesynchronization signal by the respective base station; and receiving therespective synchronization signal in a reserved random access channel ofthe respective base station if chip synchronization is produced.
 2. Amethod for disturbance-free operation of at least two base stations in auniversal mobile telecommunications system as claimed in claim 1,wherein, during transmission by one of the base stations in a defined,reserved in-house frequency range, the other base station first checkswhether the desired frequency is available and, if the desired frequencyis unavailable, the other base station falls back to a frequency in thereserved frequency range.
 3. A method for disturbance-free operation ofat least two base stations in a universal mobile telecommunicationssystem as claimed in claim 2, wherein, during simultaneous operation ofa plurality of base stations in a frequency range utilized to capacity,the receiving base station selects a minimally interfering frequency andproduces chip synchronization with respect to at least one of aninterference-causing base station and a nearest base station.
 4. Amethod for disturbance-free operation of at least two base stations in auniversal mobile telecommunications system as claimed in claim 1, themethod further comprising the step of: performing a secondarysynchronization, after a conclusion of the autosynchronizationprocedure, with respect to a respective mobile station and a connectionset up for transmission and exchange of messages.
 5. A method fordisturbance-free operation of at least two base stations in a universalmobile telecommunications system as claimed in claim 4, the methodfurther comprising the steps of: performing a channel estimation betweenthe mobile station and the base stations using a transmitted midamble;and using a channel transfer function resulting from the channelestimation for determining and eliminating interference signals.
 6. Amethod for disturbance-free operation of at least two base stations in auniversal mobile telecommunications system as claimed in claim 1,wherein the reserved synchronization channel is a downlink channel andthe reserved random access channel is an uplink channel, the twochannels being alternately transmitted at a periodic interval.
 7. Amethod for disturbance-free operation of at least two base stations in auniversal mobile telecommunications system as claimed in claim 1,wherein further base stations are synchronized in atiming-pattern-conforming manner, and synchronization channels of thefurther base stations have a highest received field strength.
 8. Amethod for disturbance-free operation of at least two base stations in auniversal mobile telecommunications system as claimed in claim 1,wherein each respective base station has an internal random numbergenerator which initiates, for the respective base station, one of afunction exchange and an allocation exchange of the time slots for thesynchronization channel and the random access channel, allowing forrenewed synchronization even if the synchronization reference stationfails.
 9. A method for disturbance-free operation of at least two basestations in a universal mobile telecommunications system as claimed inclaim 1, wherein, in a predetermined channel, the base stations transmitinformation about CDMA codes already used, the information to be used bythe participating mobile station and base stations for minimizing theinterferences.